This invention relates to the prevention of scorching prior to crosslinking of a peroxide or azo compound crosslinkable thermoplastic and/or elastomeric composition.
A major difficulty in using organic peroxides or azo compounds in crosslinking (curing) elastomeric and thermoplastic materials applications is that they may initiate premature crosslinking (i.e. scorch) during compounding and/or processing prior to the actual phase in the overall process when curing is desired. With conventional methods of compounding, such as milling, Banbury, or extrusion, scorch occurs when the time-temperature relationship results in a condition where the peroxide or azo initiator undergoes thermal decomposition, initiating the crosslinking reaction whereby gel particles in the mass of the compounded polymer may be formed. The presence of these gel particles leads to inhomogeneity of the final product.
Excessive scorch reduces the plastic properties of the material so that it can no longer be processed, resulting in the loss of the entire batch.
Therefore, it has been widely accepted that the peroxide of choice must have a high enough activation temperature so that compounding and/or other processing steps can be successfully completed prior to the final curing step. Thus one method of avoiding scorch is to use an initiator that is characterized by having a high 10 hour half-life temperature. The disadvantage to this approach is that one subsequently obtains a longer cure time, which results in lower throughput. High cure temperatures can be used but this runs into the disadvantage of higher energy costs.
A further way of avoiding scorch is to lower the compounding and/or processing temperature to improve the scorch safety margin of the crosslinking agent. This option however may be somewhat limited in scope depending upon the polymer and/or process involved. In addition, curing at the lower temperature requires longer cure times and results in lower throughput. Prior to the present invention, certain additives were incorporated into compositions which reduced the tendency for scorching. For example, British patent 1,535,039 discloses the use of organic hydroperoxides as scorch inhibitors for peroxide-cured ethylene polymer compositions. U.S. Pat. No. 3,751,378 discloses the use of N-nitroso diphenylamine or N,Nxe2x80x2-dinitroso-paraphenylamine as retarders incorporated in a polyfunctional acrylate crosslinking monomer for providing long Mooney scorch times in various elastomer formulations. U.S. Pat. No. 3,202,648 discloses the use of nitrites such as isoamylnitrite, tert-decyl nitrite and others as scorch inhibitors for polyethylene. U.S. Pat. No. 3,954,907 discloses the use of monomeric vinyl compounds as protection against scorch. U.S. Pat. No. 3,335,124 describes the use of aromatic amines, phenolic compounds, mercaptothiazole compounds, bis(N,N-disubstituted thiocarbonyl)sulfides, hydroquinones and dialkyldithiocarbamate compounds. The use of mixtures of the active compounds in preventing scorch is neither taught nor suggested. U.S. Pat. No. 4,632,950 discloses the use of mixtures of two metal salts of disubstituted dithiocarbamic acid, wherein one metal salt is based on copper. This reference does not teach the use of such mixtures with neat peroxides. For some applications, it is desirable or mandatory to use liquid or neat peroxides, as described in this current invention. One such application is in extruded compounding. A common commercial process technique employs a liquid peroxide which is sprayed onto polymer pellets or granules to coat them prior to extrusion compounding. This can provide increased production efficiency and eliminates physical handling of hazardous compounds. This reference patent teaches that at least one filler must be present. The scorch resistant systems described in this reference are not effective in polyolefins specifically LDPE, LLDPE, or HDPE. The present invention is effective in polyolefin systems. Moreover, this reference does not teach the use of mixtures of hydroquinones and metal salts of disubstituted dithiocarbamic acid.
When employing these prior art methods for extending scorch time, the cure time and/or final crosslink density of the cured composition can be adversely affected, leading to a decrease in productivity and/or product performance. The present invention overcomes the disadvantages of the prior art in that an improvement in scorch at compounding temperatures is achieved without significant impact on the final cure time or crosslink density. This is achieved by incorporation of the cure retarding composition at low additive levels, thereby limiting the effect on properties. In addition, significant scorch protection is achieved, since the use of the combination of the hydroquinones and a sulfur accelerator of the dithiocarbamate or thiuram class results in a synergistic effect on scorch time at the low additive levels employed.
The present invention provides in a first composition aspect a scorch retarding composition comprising a hydroquinone and at least one sulfur accelerator.
The tangible embodiments of this composition aspect of the invention possess the inherent applied use characteristics of being scorch retarders showing greater effect than equivalent amounts of either component used separately when incorporated into polymeric compositions which are crosslinkable by free radical initiation while not substantially affecting final cure time or properties.
Special mention is made of compositions of the first composition aspect of the invention which additionally comprise a coagent.
The invention also provides in a second composition aspect a scorch retarding, curing/crosslinking composition comprising a free radical initiator selected from the group consisting of organic peroxides, azo compounds and mixtures thereof, and the scorch retarding composition of the first composition aspect of the invention.
The tangible embodiments of this second composition aspect of the invention possess the inherent applied use characteristics, when blended into conventional thermoplastic and/or elastomeric polymers as a crosslinking agent, of providing improved scorch protection for the blended system while not substantially affecting final cure times or characteristics.
This invention also provides in a third composition aspect a crosslinkable composition comprising a peroxide or azo compound crosslinkable thermoplastic and/or elastomeric polymer, and a scorch retarding curing/crosslinking composition as defined in the second composition aspect of the inventions.
The invention also provides in an improved process for the preparation of a crosslinkable composition comprising a peroxide or azo compound crosslinkable thermoplastic and/or elastomeric polymer and a free radical initiator selected from the group of organic peroxides, azo compounds and mixtures thereof wherein said polymer is compounded with said free radical initiator, the improvement comprising performing said compounding in the presence of a scorch retarding composition of the first composition aspect of the invention.
Special mention is made of processes of this process aspect of the invention wherein the scorch retarding composition additionally comprises a coagent.
In the practice of this invention, the preferred blends of hydroquinones and sulfur accelerators exhibit acceptable solubility in the free radical initiators when the selected free radical initiator is a liquid or low melting solid. Thus, this new technology will allow for a pumpable or a meterable homogeneous crosslinking system that provides ease of handling and greater worker safety as well as longer compounding times for better mixing due to the improved scorch protection provided.
Where homogenous liquid or low melting solid crosslinking compositions are not normally used such as in rubber compounding, and the selected scorch retarding crosslinking composition is liquid, the hydroquinone, peroxide, sulfur accelerator and optional coagent(s) either as individual portions, or the entire combined scorch retarding crosslinking composition may be dispersed on an inert filler (preferably an inorganic filler) for ease of addition during compounding such as on a rubber mill. A masterbatch on a polymeric binder may be used in the same fashion for the same purpose.
The superior scorch resistance for peroxide and azo crosslinkable elastomeric and/or thermoplastic polymeric systems may be obtained by admixing, conveniently by employing conventional compounding means, with the thermoplastic and/or elastomeric polymer which is desired to be crosslinked, a scorch retarding crosslinking composition comprising a free radical initiator selected from the group consisting of organic peroxides, azo compounds and mixtures thereof, a hydroquinone compound, at least one sulfur accelerator, and optionally any of the known acrylic, methacrylic or allylic monomers.
The scorch retarding curing/crosslinking composition may preferably be blended into the desired polymer as a preformed mixture or the individual ingredients thereof may be incorporated into the polymer separately or even as subcombinations of one or more but not all the ingredients. If incorporation as individual or subcombinations of ingredients is desired, it is preferred that the hydroquinone, monomers, and/or the sulfur accelerator be blended into the polymer prior to blending of the free radical initiator.
Free Radical Initiators
In accordance with the present invention, compounds well known in the art such as azo initiators and/or organic peroxides (with the exception of hydroperoxides and peroxydicarbonates) which upon thermal decomposition generate free radicals that facilitate the curing/crosslinking reaction may be employed. Of the free radical initiators used as crosslinking agents, the dialkyl peroxides and diperoxyketal initiators are preferred. A detailed description of these compounds may be found in the Encyclopedia of Chemical Technology, 3rd edition, Vol. 17, pp 27-90. (1982)
In the group of dialkyl peroxides, the preferred initiators are:
dicumyl peroxide
di-t-butyl peroxide
t-butyl cumyl peroxide
2,5-dimethyl-2,5-di(t-butylperoxy)-hexane
2,5-dimethyl-2,5-di(t-amylperoxy)-hexane
2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3
2,5-dimethyl-2,5-di(t-amylperoxy)hexyne-3
alpha,alpha-di[(t-butylperoxy)-isopropyl]-benzene
di-t-amyl peroxide
1,3,5-tri-[(t-butylperoxy)-isopropyl]benzene
1,3-dimethyl-3-(t-butylperoxy)butanol
1,3-dimethyl-3-(t-amylperoxy)butanol and mixtures thereof.
In the group of diperoxyketal initiators, the preferred initiators are:
1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane
1,1-di(t-butylperoxy)cyclohexane
n-butyl 4,4-di(t-amylperoxy)valerate
ethyl 3,3-di(t-butylperoxy)butyrate
2,2-di(t-amylperoxy)propane
3,6,6,9,9-pentamethyl-3-ethoxycarbonylmethyl-1,2,4,5-tetraoxacyclononane;
n-butyl-4,4-bis(t-butylperoxy)-valerate;
ethyl-3,3-di(t-amylperoxy)-butyrate and mixtures thereof.
Other peroxide, e.g., 00-t-butyl-0-hydrogen monoperoxysuccinate; 00-t-amyl-0-hydrogen-monoperoxysuccinate and/or azo initiators e.g., 2,2xe2x80x2-azobis-(2-acetoxypropane) may also be used to provide a crosslinked polymer matrix. Mixtures of two or more free radical initiators may also be used together as the initiator within the scope of this invention.
Other suitable azo compounds include those described in U.S. Pat. No. 3,862,107 and 4,129,531 which are incorporated herein by reference.
The amount of the scorch retarding crosslinking composition aspect of this invention to be incorporated in a crosslinkable composition will readily be selected by one of skill in the art to be sufficient to afford the desired degree of crosslinking. When the free radical initiator component is an organic peroxide, the scorch retarding crosslinking composition may be employed in quantities to provide a concentration of peroxide in the crosslinkable composition ranging from 0.01 to 30 parts by weight, preferably, from 0.01 to 20 parts by weight, most preferably from 0.5 to 4.0 parts by weight for each 100 parts by weight of polymer.
Sulfur Accelerators
Any of the known sulfur accelerators as understood by one of skill in the art to be employed in curing of elastomers are contemplated for use in the practice of the invention. One sulfur accelerator class that is suitable for use in the practice of this invention comprises metal salts of disubstituted dithiocarbamates. The metal salts of disubstituted dithiocarbamic acid, which are suitable in the practice of this invention may be represented by the structure: 
wherein X is an ion derived from a metal selected from the group consisting of nickel, cobalt, iron, chromium, tin, zinc, copper, lead, bismuth, cadmium, selenium, and tellurium, n may vary from 1 to 6 and is equal to the formal valence of the metal, R1 and R2 are independently alkyl of 1 to 7 carbon atoms.
Examples of the metal salts of disubstituted dithiocarbamic acid are:
bismuth dimethyldithiocarbamate
cadmium diamyldithiocarbamate
cadmium diethyldithiocarbamate
copper dimethyldithiocarbamate
lead diamyldithiocarbamate
lead dimethyldithiocarbamate
selenium dimethyldithiocarbamate
tellurium diethyldithiocarbamate
zinc diamyldithiocarbamate
zinc diethyldithiocarbamate
zinc dimethyldithiocarbamate
selenium dimethyldithiocarbamate
A second sulfur accelerator class that is also suitable for use in the practice of this invention comprises the thiurams. Thiuram accelerators are prepared from secondary amines and carbon disulfide. They may be represented by the following structure: 
wherein R3 is an alkyl group of 1 to 7 carbon atoms and n may have a positive value from greater than zero up to 6. Examples of thiuram type accelerators include:
tetrabutylthiuram disulfide
tetraethylthiuram disulfide
tetramethylthiuram disulfide
tetramethylthiuram monosulfide
These classes of sulfur accelerators as well as other suitable classes of sulfur accelerators such as the sulfenamides, thiazoles, thioureas and xanthates are described in further detail in The Vanderbilt Rubber Handbook, pp 339-380. The sulfur accelerators described therein encompass the classes of sulfur compounds which would be comprehended by one of skill in the art of curing elastomeric polymers as sulfur accelerators. Simple mercaptans of the formula RSH are not included in this class of sulfur accelerators.
Hydroquinones
The hydroquinones which are suitable in the practice of this invention are described in detail in the Encyclopedia of Chemical Technology, Third Edition, vol. 19 pp 572-606. Examples of hydroquinones particularly useful in the practice of this invention are:
hydroquinone
hydroquinone di(beta-hydroxyethyl)ether
hydroquinone monomethyl ether
mono-tert-butyl hydroquinone
di-t-butyl hydroquinone
di-t-amyl hydroquinone
The sulfur accelerator and the hydroquinone are employed in amounts that are sufficient to achieve the desired balance in cure characteristics. The weight ratio of hydroquinone compound to sulfur accelerator is from 1:50 to 500:1 preferably from 1:25 to 250:1 more preferably from 1:25 to 25:1, still more preferably from 1:10 to 10:1 and most preferably from 1:1 to 5:1. The weight ratio of this blend to peroxide can range from 0.5:100 to 1:2, preferably from 1:100 to 1:2, more preferably from 1:100 to1:4 and still more preferably from 1:25 to 1:20.
Coagents
Various vinyl and/or allyl monomers are used to enhance crosslinking and as such are often called crosslinking coagents. The effective coagents are generally difunctional or polyfunctional vinyl and/or allyl monomers.
The use of these monomers or crosslinking coagents in the practice of this invention provides a number of advantages:
1) The extent of crosslinking as measured by MH, the maximum torque shown by an oscillating disc rheometer is enhanced or maintained in the final cured polymer when scorch retarding compared with formulations not employing coagents.
2) The solubility and ease of preparation of solutions of the peroxide, quinone and sulfur accelerator are surprisingly facilitated;
3) An important and unexpected enhanced phase and color stability is provided in scorch retarding curing/crosslinking peroxide solution formulations contemplated by the second composition of the invention.
4) It has surprisingly been found, for those compositions tested, when ingredients are combined in the proper order, the speed and ease of dissolution and thus the preparation of the second composition aspect compositions of the invention are made more rapid and easier. This order is first coagent, second hydroquinone, third sulfur accelerator, last peroxide or azo compound.
Blends of coagents may also be used in the practice of this invention wherein monofunctional monomers may be used in combination with the di- or poly-vinyl and/or allyl monomers.
Representative monomers include but are not limited to the following: methyl methacrylate, lauryl methacrylate, allyl methacrylate, trimethylol propane triacrylate, triallyl cyanurate, triallyl isocynaurate, triallyl phosphate, tetraallyloxyethane, allyldiglycol, carbonate, triallyltrimellitate, triallylcitrate, diallyl adipate, diallylterephthalate, diallyl oxalate, diallyl fumarate, ethylene glycol dimethacrylate, 2-hydroxyethyl methacrylate.
Other polyfunctional vinylic compounds such as liquid 1,2-polybutadiene may also be used.
Particular preferred monomers are selected from: allyl methacrylate, triallylcyanurate, triallyltrimellitate, triallylisocyanurate, allydiglycolcarbonate, diallyl oxalate, methyl methacrylate and blends thereof.
The monomeric compounds, when incorporated into any of the composition aspects of the invention, may be used in ratios of 100:1 to 1:100 preferably 50:1 to 1:50, most preferably 10:1 to 1:10 with respect to the combined amount of sulfur accelerators and quinones present.
Polymers
The thermoplastic and/or elastomeric polymers encompassed in the present invention may be defined as those natural or synthetic polymers which are thermoplastic and/or elastomeric in nature, and which can be crosslinked (cured) through the action of a crosslinking agent. Rubber World, xe2x80x9cElastomer Crosslinking with Diperoxyketals,xe2x80x9d October, 1983, pp.26-32, and Rubber and Plastic News, xe2x80x9cOrganic Peroxides for Rubber Crosslinking,xe2x80x9d Sep. 29, 1980, pp. 46-50, describe the crosslinking action and crosslinkable polymers. Polyolefins suitable for use in this invention are described in Modern Plastics Encyclopedia 89 pp 63-67, 74-75. Illustrative polymers include linear low density polyethylene, low density polyethylene high density polyethylene, chlorinated polyethylene, ethylene-propylene terpolymers, ethylene vinyl acetate ethylene-propylene copolymers, silicone rubber, chlorosulfonated polyethylene, fluoroelastomers.
In addition, blends of two or more polymers may be employed. The polymers described above and the crosslinkable compositions prepared therefrom may contain various other additives known to those skilled in the art including fillers such as carbon black, titanium dioxide, and the alkaline earth metal carbonates. Monomeric co-agents such as triallylcyanurate, allyldiglycolcarbonate, triallylisocyanurate, trimethylolpropane diallylether, trimethylolpropane trimethacrylate, various allylic compounds, methacrylates and acrylate compounds may also be added separately to the various polymers above. It is also well known in the art that polymer containing compositions in general may also contain antioxidants, stabilizers, plasticizers, and processing oils. The crosslinkable compositions of this invention may also contain such conventional additives.
The novel compositions can be incorporated into a masterbatch or carrier comprising various polyolefins and/or elastomers at levels from about 5 to 80 percent by weight.
For ease of addition for certain processes, the scorch retarding crosslinking composition, in the form of a homogenous liquid or meltable solid, may be dispersed on an inert filler such as CaCO3, silica or clay at levels from about 10 to 80 percent by weight.
The scorch retarding crosslinking composition can be incorporated into a polymeric thermoplastic and/or elastomeric material, as a preformed mixture or with the addition of each component separately, resulting in improved scorch protection. The weight ratio of hydroquinone compound to sulfur accelerator in the first composition aspect of the invention may be from 1:50 to 500:1, preferably from 1:25 to 250:1, more preferably from 1:25 to 25:1, still more preferably from 1:10 to 10:1, and most preferably from 1:1 to 5:1. The weight ratio of the first composition aspect to peroxide in the second composition aspect of the invention may range from 0.5:100 to 1:2, preferably from 1:100 to 1:2, more preferably 1:100 to 1:4, and still more preferably 1:25 to 1:20. The peroxide, quinone, sulfur accelerator and optional coagent containing second composition aspect of the invention may be incorporated into the polymeric thermoplastic and/or elastomeric material in quantities to provide a peroxide concentration in the crosslinkable composition ranging from 0.01 to 30 parts by weight, preferably from 0.01 to 20 parts by weight, most preferably from 0.5 to 4.0 parts by weight for each 100 parts by weight of polymer.
The crosslinkable composition may be heat cured to a time sufficient to obtain the desired degree of crosslinking. The heat curing has a temperature-time relationship which is primarily dependent on the polymeric compound and the peroxide initiator present, but that relationship may be affected by other ingredients in the formulation. It is customary to use a time equal to about 6 to 8 half-lives of the initiator, but this may be varied based on experience at the option of the operator depending on the exact properties desired in the final product. The inclusion of the scorch retarding compositions of this invention has no substantial effect on the time-temperature relationship when compared to the relationship in a similar system without the scorch retarding composition.
Crosslinking (curing) may be carried out at a temperature of 100xc2x0-300xc2x0 C. or more. The cure time is inversely related to the temperature. Systems employing the preferred initiators heat cure at temperature-time relations of about 120xc2x0-200xc2x0 C. and 0.5 to 30 minutes. The heat curing may be carried out in any conventional fashion such as mold cures, oil bath cures (where oil does not harm the polymeric compound), oven cures, steam cures, or hot metal salt bath cures.
All formulations were compounded utilizing the C.W. Brabender Plastigraph with type-5 mixing blades Mixer temperatures are specified below for various resin types.
To prepare crosslinkable compositions, except for the polymer, all components of the composition, for example, the peroxide, a disubstituted dithiocarbamic acid, and hydroquinone were weighed at the desired parts by weight resin into a ten dram vial and mixed to form a homogeneous solution. The quantity of each ingredient expressed in parts per 100 parts of polymer is listed in each example.
For both thermoplastic and rubber (elastomeric) compositions, 100 parts by weight of polymer were fluxed in the mixer using a mixing speed of 30 rpm at a mixing temperature designated in the specific examples. The preweighed component mixture in the vial was then slowly added to the fluxing resin. The composition was then allowed to mix for six (6) minutes, after which the composition was removed and subsequently pressed into a flat plaque (of no specific thickness), using a Carver laboratory press (Model C) set at the polymer melting point, folded and pressed at least six times to remove air bubbles and smooth out sample, and then the plaque was allowed to cool to room temperature.
Testing Crosslinking evaluations were carried out on the prepared compositions using a Monsanto Oscillating Disk Rheometer (Model R-100).
The Monsanto Rheometer test procedure consists of an uncured sample enclosed, under positive pressure, in a heated die cavity containing a biconical disk. The disk is oscillated (100 cycles/min) through an arc of 1xc2x0 or 3xc2x0 or 5xc2x0. The force, or torque, required to oscillate the disk is recorded as a function of time. This shear modulus is proportional to the extent of crosslinking, and is a representation of the cure reaction. The shear modulus increases as percent crosslinking increases. The test variables recorded from the rheometer were:
MHxe2x80x94Maximum torque (in-lbs), a measure of crosslinking attained.
MLxe2x80x94Minimum torque (in-lbs), a measure of viscosity of the compound and an indicator of scorch. Increased ML values are indicative of scorch.
MHxe2x88x92MLxe2x80x94Difference between maximum and minimum torque values. This is useful in determining extent of crosslinking.
TC90xe2x80x94Cure Time (minutes), time to reach 90% of maximum torque as defined by (MHxe2x88x92ML) 0.9+ML.
TS2xe2x80x94Scorch time (minutes), time required for torque to increase two inch-pounds above ML 
TVxe2x80x94Vulcanization time, calculated by TC90xe2x88x92TS2, a measure of cure rate, in which the curing rate is isolated from the scorch or processing phase.
xcex94TS2xe2x80x94Delta TS2 (minutes), the difference in scorch time calculated by the TS2 of a scorch retarded peroxide containing polymer formulation minus the TS2 for a comparable reference or control peroxide containing polymer formulation. The cure is adjusted so that (MH) is virtually identical for both formulations.
Other reported xe2x80x9cDeltaxe2x80x9d (xcex94) values have been determined in similar fashion from the differences determined for the particular variable.
Torque values reported (MHxe2x88x92ML) are rounded off to the nearest whole number. Scorch time values are rounded off to the nearest tenth of a minute.
The following examples are provided to illustrate preferred embodiments of the invention, and are not intended to restrict the scope thereof.